The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Electric vehicles and hybrid vehicles typically include a battery system and an electric motor that are used to propel the vehicle. The battery systems typically include a plurality of battery cells that are connected together. During operation of the vehicle, the battery cells are discharged and may be recharged by a utility and/or during operation using regenerative braking.
Battery systems for electric vehicles may cost on the order of $5000 or more, which makes the battery system one of the most expensive components of the vehicle. Given the cost of replacement, the service life of the battery system should be as long as possible. Battery life for most types of battery cells is related to the number of charge/discharge cycles and the depth of discharge. For example only, for Lithium Ion (LiIon) battery cells, the estimated number of cycles is equal to Cycles=20·106·DOD−2, where DOD is depth of discharge as a percentage. When operating a LiIon battery cell between 100% and 15%, the estimated number of cycles is 2768. When operating a LiIon battery cell between 100% and 20%, the estimated number of cycles is 3125, which is approximately 11% greater cycle life as compared to discharging to 15%. Thus, controlling the battery discharge level can impact service life.
Control systems may set a target discharge level to a desired value such as 20% (rather than a lower level such as 15%) to receive the benefit of the longer service life. The battery system will need to be replaced when one of the battery cells in the battery system reaches the end of its service life. If the control system is not accurate in maintaining the desired discharge level for each battery cell, one or more of the battery cells may be regularly discharged to a lower state of charge (i.e. below 20%) than others of the battery cells, which will prematurely end the service life of the battery system. The remaining service life of the other battery cells that were not discharged below the target discharge level will be wasted.